1. Field of the Invention
The present invention relates to the transmission of data between two locations using a radio frequency carrier, and, more particularly, relates to the transmission of data from an orbiting satellite to the ground and a technique for mitgating the effect of heavy precipitation on transmissions between the orbiting satellite and an earth station when heavy precipitation causes attenuation of the signal transmitted from the satellite.
2. Description of the Related Art
In domestic satellite communication systems, the Ku band, corresponding to frequencies in the range of 12-14 gigahertz, is widely used to transmit digital data signals from an orbiting satellite to a plurality of ground stations. Although the satellite is orbiting at a height such that it is virtually unaffected by weather conditions on the earth, the signals which propagate between the satellite and the ground stations must travel through the earth's atmosphere and are thus subjected to the weather conditions that exist along the line of sight between the satellite and any particular ground station. A major drawback associated with the use of the Ku band for communications between the ground stations and an orbiting satellite is that the signals in the 12-14 GHz range are attenuated by rainfall. For example, the passage of an active cumulonimbus storm cell across the line of sight between a ground station and the satellite can cause sufficient attenuation such that the ground station is no longer able to detect and discriminate the modulated signal being transmitted by the satellite. This attenuation is called "rain fading"; however, it can result from snow, sleet, or other atmospheric conditions. Although the same effect will occur for signals transmitted from the ground station to the satellite, typically a ground station is not power limited, and thus the attenuation can be overcome by increasing the power in the signal transmitted by the ground station. On the other hand, an exemplary satellite typically has a limited amount of power available to operate its transmitter and associated circuitry. Thus, it is generally not feasible to overcome the attenuation caused by precipitation by simply increasing the transmitted power of the satellite. For example, if precipitation is causing a 10 dB attenuation in the signal received by a particular ground station, the power in the signal transmitted by the satellite would have to be increased by 10 dB in order to overcome the attenuation. This corresponds to a ten-fold increase in the power requirements of the satellite (i.e., from 5 watts to 50 watts). To increase the power by this amount may require more than a ten-fold increase in cost. Furthermore, since it is likely that only one or a few of the ground stations are affected by rain-induced attenuation at any given time, the increase in the transmitted power would result in the unaffected ground stations receiving ten times the power which they need to receive. Thus, simply increasing the transmitted power of the satellite is not a desired solution to the problem.
Certain techniques have developed for overcoming the problem of rain fading. For example, U.S. Pat. No. 4,309,764, to Acampora, describes a technique in which the digital data transmitted from the satellite to the ground stations using a Time Division Multiple Access (TDMA) technique. The transmitted signal includes spare TDMA time slots in each frame sequence that are shared among all the ground stations and which can be allocated to a ground station experiencing rain fading. For example, a ground station experiencing rain fading may be provided with the equivalent of three additional time slots so that the data is transmitted in an encoded form that can be more readily detected in the presence of a high level of attenuation. The extra time slots require the frame sequence to be longer and thus this method causes a decrease in the overall data rate of the transmitted data.
In U.S. Pat. No. 4,287,598, to Langseth, et al., a system is described which provides land communication paths between the ground stations. In the event the signal from the satellite to a particular ground station is attenuated to the point of terminating the communication between the ground station and the satellite, the communication is redirected to a second ground station that is located outside the area of the fade condition and is then transmitted to the affected ground station via a land communication path. This, of course, partially eliminates one of the advantages of having a satellite communication system in that a land communication system is required to maintain communications. U.S. Pat. No. 3,896,382, to Magenheim, discloses a similar technique for rerouting the transmission from the satellite through an alternative ground station (FIG. 1) separated from the primary ground station by a relatively large distance. In the alternative, the Magenheim patent shows the transmission from the ground station to a second satellite at a different angle that avoids the rain cell causing the attenuation. Both of these techniques require additional equipment and are thus costly to implement.
U.S. Pat. No. 3,676,778, to Mori, discloses a satellite communication system in which the receivers and transmitters operate at a plurality of frequency bands having differing attenuations in the presence of precipitation. Mori describes a technique that mitigates the rain fading problem by assigning frequency bands having low attenuation to precipitation to those ground stations incurring attenuation at the primary frequency band, while maintaining communication at the primary frequency for the unaffected ground stations This, of course, requires the satellite to have at least two transmitters for transmitting on the two (or more) frequency bands. This requires additional hardware in the satellite and also requires additional power to operate the second transmitter.
A need continues to exist for a technique for communicating between a satellite and a ground station that provides a means for mitigating the effect of attenuation of the signal caused by precipitation in the line of sight between the satellite and the ground station without the need for ground communication channels or alternate satellite to ground signal paths.